1. Field of the Invention
Embodiments of the invention relate to electronic devices, and more particularly, in one or more embodiments, to an interconnection layout for integrated circuits and/or printed circuit boards.
2. Description of the Related Art
In many applications in which electronic information is transmitted over a relatively long line, differential signaling has been widely used. Differential signaling is a method of transmitting information using two complementary signals sent on two separate lines. A differential circuit at a receiving end detects and compares the complementary signals, and determines logic changes based on the changes of one of the signals with reference to the other. Differential signaling is known to provide a relatively fast and accurate data transmission mechanism.
In differential signaling, however, a pair of lines carrying complementary signals can have electrical coupling or cross-talk between the pair of lines (intra-pair coupling) and/or with another neighboring pair of lines (inter-pair coupling). This electrical coupling adversely affects the accuracy of information transmitted over the lines. Thus, there is a need to provide a scheme to reduce or eliminate electrical coupling among separate pairs of differential lines.
FIG. 1 illustrates a conventional interconnection layout that can be used in an integrated circuit (IC) or a printed circuit (PC) board (also known as a printed wiring board) for differential signaling. The illustrated portion of the layout 100 can be repeated vertically and/or horizontally in the IC or PC board.
Typically, an interconnection layout for differential signaling can include a pair of lines that are “twisted” (wound), cross back and forth without twisting, or a combination of both. The illustrated portion includes first to fourth differential pairs L1-L4. Each of the differential pairs L1-L4 includes two lines carrying differential signals. In FIG. 1, the first pair L1 includes first and second lines L1a, L1b. The second pair L2 includes first and second lines L2a, L2b. The third pair L3 includes first and second lines L3a, L3b. The fourth pair L4 includes first and second lines L4a, L4b. The illustrated portion of the layout 100 includes four regions a, b, c, d from left to right. A boundary 121, 122, or 123 between neighboring ones of the regions a, b, c, d extends substantially perpendicular to a direction in which the pairs L1-L4 of lines extend. Each of the four regions a, b, c, d includes portions of all the pairs L1-L4 of lines.
Each of the pairs L1-L4 of lines includes crossing portions CP at an interval of ½ l. The crossing portions CP of the lines cross each other, for example, forming an “X” shape. The details of the crossing portions CP will be described later with reference to FIGS. 3A-3C and 4A-4C. Each of the pairs L1-L4 of lines includes parallel portions PP between neighboring ones of the crossing portions CP. The parallel portions PP of the lines extend substantially parallel to each other.
In FIG. 1, odd-numbered pairs L1, L3 have crossing portions CP adjacent to the parallel portions PP of neighboring even-numbered pairs L2, L4. Similarly, even-numbered pairs L2, L4 have crossing portions CP adjacent to the parallel portions PP of neighboring odd-numbered pairs L1, L3. The crossing portions CP of the first and third pairs L1, L3 are positioned at the boundary 122 between the regions b and c. Some of the crossing portions CP of the second and fourth pairs L2, L4 are positioned at the boundary 121 between the regions a and b while the other crossing portions CP of the second and fourth pairs L2, L4 are positioned at the boundary 123 between the regions c and d.
In the layout 100 of FIG. 1, the line L2a is adjacent to the line L1b in the region a, the line L3a in the region b, the line L3b in the region c, and the line L1a in the region d. The line L2b which pairs with the line L2a is adjacent to the line L3a in the region a, the line L1b in the region b, the line L1a in the region c, and the line L3b in the region d. Thus, within a length of l, both of the paired lines L2a, L2b experience electrical coupling with each of the lines L1a, L1b, L1a, L3b of the neighboring pairs L1, L3 by ¼ l. Because signals on the paired lines L1a, L1b are opposite in polarity to each other, coupling induced on the line L2a by these lines L1a, L1b are also opposite in polarity. Thus, coupling between the line L2a and the adjacent first pair L1 is canceled or reduced. Likewise, coupling between the line L2a and the other adjacent third pair L3 is canceled or reduced. Similarly, coupling between the line L2b and the first pair L1 and coupling between the line L2b and the third pair L3 are also canceled or reduced. In this manner, the layout 100 cancels or reduces inter-pair coupling.